Patent Literatures 1 and 2 discuss a projector that uses a LED (Light Emitting Diode) as a light source for illuminating a DMD (Digital Micromirror Device) serving as a display element. The projector of this type is expected to reduce costs more than a projector that uses a white discharge lamp, and achieve performance including miniaturization, lower power consumption, and a wider color reproduction range.
There are also a projector that uses a single LCoS (Liquid Crystal on Silicon) element (Nonpatent Literature 1), and a projector that users three liquid crystal panels as display elements (Nonpatent Literatures 2 and 3).
In the liquid crystal projector, usually, a liquid crystal panel must be illuminated with polarized light. Generally, light from the LED is unpolarized light. Thus, when the LED is used as a light source for illuminating the liquid crystal panel, it is preferable that the light from the LED be converted into polarized light to increase light use efficiency.
For example, when a display panel is a TN liquid crystal (Twisted Nematic Liquid Crystal) panel, with respect to the unpolarized light from the LED which includes two linear polarized components which are orthogonal each other, one of linear polarized component is converted to be similar to the other polarized component. When efficiency of this polarizing conversion is low, the light use efficiency declines. When no polarizing conversion is carried out, about a half of the light is not used as illumination light.
In the projector described in Nonpatent Literature 1, a luminous flux from the LED that is the light source is roughly converted into parallel luminous fluxes by a light guiding member having a composite paraboloidal surface shape, which is referred to as a CPC reflector. The display panel is illuminated with the parallel luminous fluxes from the light guiding member.
A ¼ wavelength plate and a reflective polarizing plate are located in the traveling direction of the luminous flux from the light guiding member. For example, the polarizing plate transmits p-polarized light while reflecting s-polarized light. The s-polarized light reflected by the polarizing plate passes through the ¼ wavelength plate. The light (circular polarized light) passed through the ¼ wavelength plate returns to the LED to be reflected on a surface of the LED. The light reflected on the surface of the LED is converted parallel by the light guiding member, and then passes through the ¼ wavelength plate again.
A portion of the s-polarized light reflected by the polarizing plate, which has passed through the ¼ wavelength plate twice in the returning process to the LED and in the process of being reflected on the surface of the LED to travel toward the polarizing plate, is converted into p-polarized light.
In addition, there is an illuminating device discussed in Patent Literature 3. This illuminating device includes: a light source; a glass rod one end surface of which light from the light source enters; condensing means for condensing a luminous flux that has exited from the other end surface of the glass rod; and a PBS (Polarizing Beam Splitter) polarizing conversion array disposed at a position where the condensing means forms a plurality of light source images.
The PBS polarizing conversion array is formed by alternately arranging first and second prisms in one direction. Each of the first and second prisms is a rectangular parallelepiped prism formed by bonding two rectangular prisms together.
The first prism includes a polarizing separation film formed on the bonded surface of the two rectangular prisms to transmit the p-polarized light while reflecting the s-polarized light, and is configured so that incident light can enter the polarizing separation film at an incident angle of about 45 degrees.
The second prism includes a reflecting film formed on the bonded surface of the two rectangular prisms, and is configured so that the s-polarized light reflected by the polarizing separation film of the first prism can enter the reflecting film at an incident angle of about 45 degrees. A surface of the second prism located in the traveling direction of the light reflected by the reflection film is an exit surface. A ½ wavelength plate for converting the s-polarized light into p-polarized light is formed on the exit surface. A light shielding plate is formed on a surface opposite the exit surface of the second prism.
The p-polarized light output from the first prism and the p-polarized light output from the second prism travel in the same direction. The condensing means is configured to form a light source image on an incident surface of the first prism.